Muscarinic acetylcholine receptors (mAChRs) are members of the G protein-coupled receptor superfamily which mediate the actions of the neurotransmitter acetylcholine in both the central and peripheral nervous system. Five mAChR subtypes have been cloned, M1 to M5. The M4 mAChR is predominantly expressed in the striatum, but also in the hippocampus and cortex; M2 mAChRs are located predominantly in the brainstem and thalamus, though also in the cortex, hippocampus and striatum where they reside on cholinergic synaptic terminals (Langmead et al., 2008 Br J Pharmacol). However, M2 mAChRs are also expressed peripherally on cardiac tissue (where they mediate the vagal innervation of the heart) and in smooth muscle and exocrine glands. M3 mAChRs are expressed at relatively low level in the CNS but are widely expressed in smooth muscle and glandular tissues such as sweat and salivary glands (Langmead et al., 2008 Br J Pharmacol).
Muscarinic receptors in the central nervous system play a critical role in mediating higher cognitive processing and control of dopamine release. Schizophrenia is a neuropsychiatric disease consisting of multiple symptom domains (positive, negative, cognitive and mood). One of the hypotheses of the disease is that various symptom domains are due to alterations in dopamine signalling, including hyperactivity of the mesolimbic dopamine pathway and hypoactivity of the mesocortical pathway. Muscarinic M4 receptors are expressed pre-synaptically on cholinergic pathways originating in the laterodorsal, subpenduncular and pendunculopontine tegmental nuclei which innervate the substantia nigra and ventral tegmental area (and control dopamine release in the striatum and nucleus accumbens). The absence of the M4 receptor in the KO mouse causes an increase in dopamine efflux in the nucleus accumbens. M4 receptors are also thought to regulate dopamine transmission in the mesocortical pathway.
Furthermore, preclinical studies have suggested that mAChR agonists display an atypical antipsychotic-like profile in a range of pre-clinical paradigms. The mAChR agonist, xanomeline, reverses a number of dopamine driven behaviours, including amphetamine induced locomotion in rats, apomorphine induced climbing in mice, dopamine agonist driven turning in unilateral 6-OH-DA lesioned rats and amphetamine induced motor unrest in monkeys (without EPS liability). It also has been shown to inhibit A10, but not A9, dopamine cell firing and conditioned avoidance and induces c-fos expression in prefrontal cortex and nucleus accumbens, but not in striatum in rats. These data are all suggestive of an atypical antipsychotic-like profile (Mirza et al., 1999 CNS Drug Rev).
Xanomeline, sabcomeline, milameline and cevimeline have all progressed into various stages of clinical development for the treatment of Alzheimer's disease and/or schizophrenia. Phase II clinical studies with xanomeline demonstrated its efficacy versus various cognitive symptom domains, including behavioural disturbances and hallucinations associated with Alzheimer's disease (Bodick et al., 1997 Arch Neurol). This compound was also assessed in a small Phase II study of schizophrenics and gave a significant reduction in positive and negative symptoms when compared to placebo control (Shekhar et al., 2008 Am J Psych). However, in all clinical studies xanomeline and other related mAChR agonists have displayed an unacceptable safety margin with respect to cholinergic side effects, including nausea, gastrointestinal pain, diarrhoea, diaphoresis (excessive sweating), hypersalivation (excessive salivation), syncope and bradycardia.
Diseases associated with cognitive impairments, such as Alzheimer's disease, are accompanied by loss of cholinergic neurons in the basal forebrain (Whitehouse et al., 1982 Science). In schizophrenia, which is also characterised by cognitive impairments, mAChR density is reduced in the pre-frontal cortex, hippocampus and caudate putamen of schizophrenic subjects (Dean et al., 2002 Mol Psychiatry). Furthermore, in animal models, blockade or lesion of central cholinergic pathways results in profound cognitive deficits and non-selective mAChR antagonists have been shown to induce psychotomimetic effects in psychiatric patients. Cholinergic replacement therapy has largely been based on the use of acetylcholinesterase inhibitors to prevent the breakdown of endogenous acetylcholine. These compounds have shown efficacy versus symptomatic cognitive decline in the clinic, but give rise to dose-limiting side effects resulting from stimulation of peripheral M2 and M3 mAChRs including disturbed gastrointestinal motility, bradycardia, nausea and vomiting (http://www.drugs.com/pro/donepezil.html; http://www.drugs.com/pro/rivastigmine.html).
Alzheimer's disease (AD) is the most common neurodegenerative disorder (26.6 million people worldwide in 2006) that affects the elderly, resulting in profound memory loss and cognitive dysfunction. The aetiology of the disease is complex, but is characterised by two hallmark brain sequelae: aggregates of amyloid plaques, largely composed of amyloid-β peptide (Aβ), and neurofibrillary tangles, formed by hyperphosphorylated tau proteins. The accumulation of Aβ is thought to be the central feature in the progression of AD and, as such, many putative therapies for the treatment of AD are currently targeting inhibition of Aβ production. Aβ is derived from proteolytic cleavage of the membrane bound amyloid precursor protein (APP). APP is processed by two routes, nonamyloidgenic and amyloidgenic. Cleavage of APP by γ-secretase is common to both pathways, but in the former APP is cleaved by an α-secretase to yield soluble APPα. The cleavage site is within the Aβ sequence, thereby precluding its formation. However, in the amyloidgenic route, APP is cleaved by β-secretase to yield soluble APPβ and also Aβ. In vitro studies have shown that mAChR agonists can promote the processing of APP toward the soluble, non-amyloidogenic pathway. In vivo studies showed that the mAChR agonist, AF267B, altered disease-like pathology in the 3×TgAD transgenic mouse, a model of the different components of Alzheimer's disease (Caccamo et al., 2006 Neuron). Finally, the mAChR agonist cevimeline has been shown to give a small, but significant, reduction in cerebrospinal fluid levels of Aβ in Alzheimer's patients, thus demonstrating potential disease modifying efficacy (Nitsch et al., 2000 Neurol).
Muscarinic agonists have also been disclosed as being useful in the treatment or management of pain, see for example WO2005/030188. The mAChR agonist, xanomeline, has shown to be active in preclinical models of both inflammatory and neuropathic pain (Martino et al., 2012, Pain).
WO2009/108117 and WO2009/034380 (both AstraZeneca) disclose 4-substituted-piperidinylpiperidine carboxylates as muscarinic receptor agonists. The data presented in the two documents indicate that the compounds are selective for the muscarinic M1 receptor and, in most cases, have little or no activity at the M4 receptor.
WO96/13262 (Merck), WO01/21590 (Schering), U.S. Pat. No. 6,294,554 (Schering) and U.S. Pat. No. 5,889,006 (Schering) each disclose 4-substituted-piperidinylpiperidines having muscarinic receptor antagonist activity.
WO98/05292 (Schering) discloses 4-substituted-piperidinylpiperidine carboxylates wherein the 4-substituent can be a phenyl-substituted saturated heterocyclic ring. The compounds are described as having muscarinic antagonist activity.
WO98/46599 (Uriach) discloses piperidinylpiperidinylthiazole carboxamides as platelet aggregation inhibitors.
WO99/32481 (Alcon) discloses piperidinylpiperidinyl carboxylates as muscarinic agents for use in treating glaucoma, myopia and other conditions.
WO2005/117883 (Vertex) discloses muscarinic receptor modulator compounds containing a bridged bicyclic group.